The present invention relates generally to variable frequency drive circuits and, more particularly, to variable frequency drive circuits with overvoltage protection.
One type of system commonly used in industry that performs power conversion is an adjustable or variable frequency drive (VFD) circuit, which is an industrial control device that provides for variable frequency, variable voltage operation of a driven system, such as an AC induction motor. One operating environment in which such VFD circuits are commonly used is in on-land and offshore drilling rigs. FIG. 1 shows an example of a such a drilling rig system 10, with the system including a 60 Hz diesel generator 12, power factor capacitors 14 coupled to the 60 Hz diesel generator 12, and a user transformer 16 coupled to the 60 Hz diesel generator 12 and the power factor capacitors 14—with the generator 12 and transformer 16 collectively providing power for the system 10. An output of the transformer 16 is provided to a point of common coupling 18 to which a number of loads are connected. For example, an AC motor 20, DC motor 22 and AC motor 24 are included in the system 10, with a thyristor bridge 26 being provided between the common coupling 18 and the DC motor 22 and a six-pulse VFD circuit 28 connected between the common coupling 18 and the AC motor 24.
It is recognized that the 60 Hz diesel generator 12 in the drilling rig system 10 is a “weak source” with a high impedance—such that the power provided to the point of common coupling 18 therefrom includes deep voltage notches, with these voltage notches at the common coupling 18 being illustrated in the waveform 30 of FIG. 2—i.e., voltage notches 32. These deep voltage notches at the point of common coupling 18—which is an input to the VFD circuit 28—may cause performance issues and component failures in the VFD circuit. To better illustrate these drawbacks, a number of typical VFD circuits 28 are illustrated in FIGS. 3 and 4—with the VFD circuits 28 including built-in or external AC input inductors 34 and electromagnetic interference (EMI) filters 36 provided between a drive input 38 and input rectifier terminals 40, a rectifier 42, a pre-charge circuit 44 including one or more relays 46 and a resistor or resistors 48, a DC link capacitor bank which may consist of one or more capacitors 50, and an inverter 52. In operation of these VFD circuits 28, the deep voltage notches 32 (FIG. 2) present in the input power provided thereto (from the “weak” power source—generator 12/transformer 16) may cause overvoltage ringing at the input rectifier terminals 40. The overvoltage ringing can cause the DC link capacitors 50 in the VFD circuit 28 to fail due to overvoltages charging the DC link capacitors 50 in an uncontrolled manner with high frequency ripple.
While solutions exist presently for addressing the problem of overvoltage ringing and associated DC link capacitor failure, these are limited to adding notch reactors at VFD circuit inputs or removing EMI filters. Each of these solutions, however, has drawbacks associated therewith—as adding notch reactors at VFD circuit inputs is expensive space consuming (i.e., the notch reactors are large) and removing EMI filters results in only a limited degree of protection being provided to the VFD circuit.
It would therefore be desirable to design a VFD circuit with a smaller and more cost effective solution for overvoltage protection.